reference request – Maslov index equal to $2$ implies that the disk is not multiply covered

In the paper Floer cohomology of lagrangian intersections and pseudo-holomorphic disks in page $1009$ the authors claim that a map $w:D^2rightarrow S^2$ with $w|_{partial D^2}subset L$, where $L$ is the equator, and such that $mu(w)=2$, where $mu$ is the maslov index of the map, cannot be multiply covered and hence it will be injective in the interior of $D^2$.

I have tried proving this , but I am getting nowhere.

Any help or reference where I can look this up is appreciated, thanks in advance.